Revert "oxcmsg: Implementation for partial UserDNs in ModifyRecipients call

This reverts commit ea9cf1980a13786ecdf17e710eaf256bee5f1152.

Unfortunately this commit brakes SOGo backend expectations that
recipient->username actually holds samAccountName for the user.
Thus although Openchange layer is doing the right thing now,
SOGo is not handling this very well. I need to work more on that.
Sorry for faulty commit